This project focuses on the role of PID1 (Phosphotyrosine Interaction Domain containing 1) in gliomas. PID1 is a phosphotyrosine binding (PTB) domain-containing protein that was discovered in 2006 and which has never been studied in the context of cancer. Ectopic expression of PID1 has opposite effects on proliferation in different cell types. Using microarrays and quantitative RT-PCR, we found that high expression of PID1 correlates with better outcome in two types of brain tumors, gliomas and medulloblastomas. Our in vitro preliminary data further show that ectopic expression of PID1 has growth-inhibitory effects in brain tumor cell lines from three different types of brain tumors: glioblastomas, medulloblastomas, and atypical teratoid rhabdoid tumors (ATRT). We chose to focus this PID1-centered R21 proposal on gliomas since they constitute the most common primary brain tumor in humans and have grim prognosis when malignant. The goal of this proposal is to uncover the molecular mechanisms by which PID1 mediates its inhibitory effect in gliomas. Specifically, we will test the hypothesis that PID1 exerts its inhibitory effect in malignant gliomas in part through its PTB domain. We propose the following two Specific Aims: 1) To determine the critical domain(s) of PID1 that mediate(s) its inhibitory function in GBM cell lines. In this Aim, we will make mutants of known and predicted domains of PID1 and test their ability to decrease proliferation and induce apoptosis. 2) To examine the effect of PID1 expression on growth of intracranial human cell line-derived GBM tumors in NOD-SCID mice. In this Aim, we will use regulated expression of PID1 and examine the effects of PID1 expression on tumor growth, survival, proliferation, and apoptosis. This work is novel and of high impact, as it will further our knowledge on the molecular mechanisms and functions of PID1 in gliomas, forming the first basis of knowledge about PID1 in any brain tumor and any cancer. The clinical correlation we uncovered in our Preliminary Data highlights the relevance and significance of this topic. In addition, our finding of an inhibitory function for PID1 in three different types of brain tumors suggest that PID1 may also be important in other cancers, and possibly in other diseases, potentially expanding the impact of our work.